1. Field of the Invention
This invention relates to a display which has special utility in connection with an attachment for a helmet worn by an aircraft pilot (helmet mounted display) wherein an image of a desired display, such as information on the face of a cathode ray tube, can be introduced into the field of view being observed by the pilot so that the desired display is superimposed on the scene being viewed in the pilot's vision.
2. Description of the Prior Art.
Helmet mounted displays are well known in the prior art and usually employ an optical system which receives an image of a desired display from a cathode ray tube and projects a collimated image onto a partially reflecting surface where it is reflected back to the pilot's eye so that the image is focused at infinity and combined with the scene being viewed by the pilot through his windshield. The apparatus is mounted onto the helmet worn by the pilot and the reflecting surface is located relatively close to one of the pilot's eyes so that the size of the optical components may be kept small. One way of accomplishing this is to employ a "folded catiodoptric" system which uses a spherical reflector that is highly reflective to the wave lengths used for the desired display and a planar or slightly curved beam splitter set at an angle to the spherical surface and which is 50 percent reflective and 50 percent transmissive. The rays from the optical system then pass to the planar beam splitter, are 50 percent reflected to the spherical surface, reflected back to the beam splitter and then 50 percent transmitted to the pilot's eye. While this concept allows the optical system to be inserted close to the pilot's eye, it does provide a problem with respect to the brightness of the desired display. Since the rays must be both reflected and transmitted through the beam splitter, a minimum loss of a factor of 4 is suffered by the display (50 percent.times.50 percent).
In recent years, the art of producing filters has advanced to a state where a filter or beam splitter may be produced which has very selective characteristics, both with respect to the wavelength of light incident upon the filter, but also with respect to the angle of incidence of the light upon the surface of the filter. More particularly, it is possible to create a filter which is tuned in such a way as to transmit only a fairly narrow wavelength of light (for example, the 540 nm to 550 nm green spike emitted by a P43 phosphor on the CRT) and also to be highly reflective when the angle of incidence of this narrow band is within a chosen band of angles while being highly transmissive to the same green light when the angles of incidence are outside the chosen range. For example, U.S. Pat. No. 4,261,647 shows an optical system referred to as a "head-up display" which has a planar and spherical surface receiving light from a CRT. The planar surface is coated so as to be reflective to a narrow band of wavelengths coming from the CRT when the rays strike the surface within a narrow conical ray bundle but which is transmissive to the CRT wavelength when the rays are outside such conical bundle. Because of the optics of the head-up display system do not need to be close to the pilot's eye, the rays from the cathode ray tube all strike the planar surface at approximately the same angle and are within the small conical bundle which become reflected to the spherical member where they are again reflected and passed back through the planar filter but now at an angle which is outside the conical bundle necessary for reflection and thus are now transmitted to the observer. Such an arrangement cannot find applicability in a helmet mounted sight since the optics are much smaller and need to be close to the pilot's eye and the rays striking the surfaces do so at a wide variety of angles. Accordingly, within the size limitations of the optical system, it is not practically possible to arrange to have the rays emerge from the optical system to the planar reflector, all at substantially the same angle and, accordingly, a filter like that of the head up display would not operate satisfactorily in a helmet mounted system.